生成器与Java语法

构建器设计模式是Java中最受欢迎的一种 。

它很简单，它有助于使对象不可变，并且可以使用Project Lombok中的@Builder或Immutables之类的工具来生成。

但是这种模式在Java中是如此方便吗？

此模板链接方法的一个示例：

public class User {

  private final String firstName;

  private final String lastName;

  User(String firstName, String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  public static Builder builder() {
      return new Builder();
  }

  public static class Builder {

    String firstName;
    String lastName;

    Builder firstName(String value) {
        this.firstName = value;
        return this;
    }

    Builder lastName(String value) {
        this.lastName = value;
        return this;
    }

    public User build() {
        return new User(firstName, lastName);
    }
  }
}

User.Builder builder = User.builder().firstName("Sergey").lastName("Egorov");

if (newRules) {
    builder.firstName("Sergei");
}

User user = builder.build();

:

1. User — , .
2. , User .
3. Builder , User .
4. this ( Builder).

… ?

, User:

public class RussianUser extends User {
    final String patronymic;

    RussianUser(String firstName, String lastName, String patronymic) {
        super(firstName, lastName);
        this.patronymic = patronymic;
    }

    public static RussianUser.Builder builder() {
        return new RussianUser.Builder();
    }

    public static class Builder extends User.Builder {

        String patronymic;

        public Builder patronymic(String patronymic) {
            this.patronymic = patronymic;
            return this;
        }

        public RussianUser build() {
            return new RussianUser(firstName, lastName, patronymic);
        }
    }
}

RussianUser me = RussianUser.builder()
    .firstName("Sergei") //  User.Builder :(
    .patronymic("Valeryevich") //   !
    .lastName("Egorov")
    .build();

, firstName :

   User.Builder firstName(String value) {
        this.value = value;
        return this;
    }

Java- , this RussianUser.Builder, User.Builder!

:

RussianUser me = RussianUser.builder()
    .patronymic("Valeryevich")
    .firstName("Sergei")
    .lastName("Egorov")
    .build() //  ! User   RussianUser
    ;

: self typing

— User.Builder , , :

 public static class Builder<SELF extends Builder<SELF>> {

    SELF firstName(String value) {
        this.firstName = value;
        return (SELF) this;
    }

RussianUser.Builder:

   public static class Builder extends User.Builder<RussianUser.Builder> {

:

RussianUser.builder()
    .firstName("Sergei") //  RussianUser.Builder :)
    .patronymic("Valeryevich") // RussianUser.Builder
    .lastName("Egorov") // RussianUser.Builder
    .build(); // RussianUser

:

class A<SELF extends A<SELF>> {

    SELF self() {
        return (SELF) this;
    }
}

class B<SELF extends B<SELF>> extends A<SELF> {}

class C extends B<C> {}

, ? … !
, !

new A<A<A<A<A<A<A<...>>>>>>>()

, ( Kotlin):

A a = new A<>();

« » (raw types) diamond operator Java. , , , .

: Self typing Java

: ( , ).
, JEP .
P.S. - , JEP? ;)

Self typing Swift.
Java-:

class A {

    @Self
    void withSomething() {
        System.out.println("something");
    }
}

class B extends A {
    @Self
    void withSomethingElse() {
        System.out.println("something else");
    }
}

new B()
    .withSomething() //    void
    .withSomethingElse();

, .
javac Self Manifold.

:

, , … ?

public class User {

  // ...

    public static class Builder {

        String firstName;
        String lastName;

        void firstName(String value) {
            this.firstName = value;
        }

        void lastName(String value) {
            this.lastName = value;
        }

        public User build() {
            return new User(firstName, lastName);
        }
    }
}
public class RussianUser extends User {

    // ...

    public static class Builder extends User.Builder {

        String patronymic;

        public void patronymic(String patronymic) {
            this.patronymic = patronymic;
        }

        public RussianUser build() {
            return new RussianUser(firstName, lastName, patronymic);
        }
    }
}

RussianUser.Builder b = RussianUser.builder();
b.firstName("Sergei");
b.patronymic("Valeryevich");
b.lastName("Egorov");
RussianUser user = b.build(); // RussianUser

« , , Java» — .
, … ?
, , ? !
:

public class User {

  // ...

    public static class Builder {
        public Builder() {
            this.configure();
        }

        protected void configure() {}

:

RussianUser user = new RussianUser.Builder() {
    @Override
    protected void configure() {
        firstName("Sergei"); //  User.Builder
        patronymic("Valeryevich"); //  RussianUser.Builder
        lastName("Egorov"); //  User.Builder
    }
}.build();

, .
«» Java: .

RussianUser user = new RussianUser.Builder() {{
    firstName("Sergei");
    patronymic("Valeryevich");
    lastName("Egorov");
}}.build();

, . Swing/Vaadin ;)

(, , ). . , , , , :

1. Java .
2. JVM-.
3. .
4. , .

, Java self typing, Java ( JVM-).

, . , .

, !

P.S. .

. Pivotal Project Reactor, JPoint (5-6 ) — Reactor, !